IOWA ACADEMY OP SCIENCE 
161 
an hour instead of four, and in place of being perhaps only a mile wide, is 
frequently several hundreds of miles in width. 
The role of the plain in the general scheme of the arid cycle is especially 
instructive; since, as Davis* with great acumen has lately noted, the latter 
is in one respect at least even better supported than that of the normal wet 
cycle, “for while the arid African plains are examples of old desert plains now 
growing older, it is difficult to point out any large peneplain that still stands 
close to the baselevel with respect to which it was worn down.” 
That some of the, low, lava-capped mesas located in the largest and dryest 
of the interment plains represent the levels of the general plains-surface at 
the time when the lava-stream was out-poured, is an inference which cannot 
well be disputed. Some of the greater successive plains-levels, as the Mesa de 
Maya, the Ocate Mesa, the Las Vegas plateau and the Canadian valley have been 
recently fully describedf. Between these great levels are many minor ones. 
On the hypothesis of normal water gradation the remarkable smoothness of 
the interment plains is very plausibly accounted for by considering them floored 
with debris brought down from the neighboring mountains. It is customary 
to regard these basin-plains as deeply filled with rock-waste from the peripheral 
highlands. In the physiographically older districts, as Russell puts it, the 
mountains appear to be buried up to their shoulders. 
In a recent discussion it is pointed outij: that in many cases at least the floors 
of these interment plains are only thinly covered by soils and that these plains 
are actually worn out on the bevelled edges of the underlying strata. Even 
around “lost mountains” as those of the Mojave desert, the tilted bed-rocks of 
the piedmont about reach the surface of the inclined plains§. 
In other piedmont districts where great thicknesses of “wash deposits” are 
reported, the unconsolidated deposits have been found, upon more critical in- 
spection, to be made up largely of Tertiary beds. This is notably the case in the 
Rio Grande valley, in some of the interment “valleys” of Arizona, and in 
Death valley in California. In the light of these observations the general 
statements that all of the interment plains of the Great Basin and other parts 
of the arid region are deeply fllled with “wash” must be held in abeyance and 
the data upon which they rest examined anew. 
The evolution of the plateau-plain thus presents some novel features. The 
governing factors in an arid cycle are discussed by Davis ||; and other phases of 
the subject I have also pointed out**. The initiation of an arid cycle may be 
considered in connection with either one of two antithetical types of relief/ 
One is a mountainous type; this is the one discussed by Davis. The other is 
the plains type, which is believed to be the type upon which the Great Basin 
is based. Beginning with an upraised peneplain the degradational processes, 
whatever might be their nature, would be expected to start to reduce 
the country towards ultimate base-level, just the same as under conditions of 
a moist climate. Assuming for the moment wind-scour to be the chief erosive 
factor instead of water-action, the broader relief features need not be very 
unlike the general topographic effects produced by stream-systems. In fancy, 
*Journal of Geology, Vol. XIII, p. 395, 1905. 
tJournal of Geology, Vol. XVII, p. 31, 1909. 
tBull. Geol. Soc. America, Vol. XIX, p. 63, 1908. 
§Trans. American Inst. Mining Eng., Vol. XL, p. 890, 1910. 
II Journal of Geology, Vol. XIII, p. 382, 1905. 
**Bull. Geol. Soc. America, Vol. XIX, p. 86, 1908. 
11 
